Wireless communication systems commonly include information-carrying modulated carrier signals that are wirelessly transmitted from a transmission source (for example, a base transceiver station) to one or more receivers (for example, subscriber units) within an area or region.
A form of wireless communication includes multiple transmit antennae and/or multiple receiver antennae. Multiple antennae communication systems can support communication diversity and spatial multiplexing.
A Wireless Channel
FIG. 1 shows modulated carrier signals traveling from a transmitter 110 to a receiver 120 following many different (multiple) transmission paths.
Multipath can include a composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and receiver. The receiver may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the receiver later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal can include a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several micro-seconds.
The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. Fading causes the modulated carrier signal to attenuate in amplitude when multiple paths subtractively combine.
Spatial multiplexing and diversity communication are transmission technologies that exploit multiple antennae at both the base transceiver station and at the subscriber units to increase the bit rate in a wireless radio link with no additional power or bandwidth consumption.
FIG. 2 shows three transmitter antenna arrays 210, 220, 230 that transmit data symbols to a receiver antenna array 240. Each transmitter antenna array and each receiver antenna array include spatially separate antennae. A receiver connected to the receiver antenna array 240 separates the received signals.
Common Amplitude and Phase Errors
Multiple channel transmitters and receivers are generally associated with spatial multiplexing or diversity signals. Multiple channel transmitters and multiple channel receivers can include multiple transmitter and receiver chains.
The multiple transmitter and receiver chains typically include amplitude noise and phase noise that vary over time. Generally, the rate in which the amplitude noise and phase noise vary is greater than a rate in which the transmission channel between the transmitter and receiver varies.
Channel training can be used to characterize the amplitude and phase noise. Channel training, however, requires a large amount of electronics overhead, and requires the transmission of a large amount of calibration information. Additionally, training is not very effective in characterizing amplitude and phase noise if the amplitude and phase noise is changing quickly.
Prior art multiple channel transmitters and multiple channel receiver generally each include a common clock that is associated with transmitter channels or the receiver channels. That is, prior art multiple chain transmitters generally include a common clock for enabling transmission from each of the multiple transmitter chains. Prior art multiple chain receivers generally include a common clock for enabling reception from each of the multiple receiver chains. Therefore, phase and amplitude errors associated with multiple chain transmitters and multiple chain receivers are generally ignored.
More advanced multiple channel wireless systems can include the transmitter chains being individually clocked, and the receiver chains being individually clocked. For example, some advanced systems include each transmitter residing at a separate base transmitter station (multiple base multiple channel system).
Multiple base spatial multiplexing or transmitter diversity systems can be much more sensitive to phase and amplitude errors than single base systems. Calibration of the phase and amplitude errors can be much more difficult because each transmitter chain is synchronized to a different clock. Multiple chain receivers having different clocks associated with each receiver chain are also difficult to calibrate.
Some wireless systems (such as mobile wireless systems and local area networks (LANs)) include interfacing transmitters and receivers that are manufactured by different companies. This can result in receivers and transmitters that include varying types of transmission and receiver chains that each influence amplitude and phase noise differently. In addition, these systems can include transmission from mobile transmitters having varied transmission channels.
It is desirable to have a method and system for calibrating phase and amplitude errors associated with transmitting and receiving multiple information signals with transmission and receiver chains that individually contribute phase and amplitude noise. The method and system should be adaptable for use with presently existing multiple channel systems without adding appreciable cost. The method and system should allow for the transmission of higher orders of modulation.